This invention relates to light emitting devices and, in particular, to organic light emitting devices.
Organic light emitting diodes are of interest for emissive flat panel displays with low, medium, or high information content for a wide range of military, industrial, consumer, and automotive applications. For virtually all applications, but particularly for portable and other low-power applications, devices with low turn-on voltage and low operating voltages are desirable.
Organic light emitting diodes are typically fabricated by sandwiching one or more appropriate organic films between two conductive electrodes. When an electric field is applied across the device, electrons are injected into the organic film from the negatively charged electrode (the cathode), and holes are injected from the positively charged electrode (the anode). The injected carriers travel through the organic material under the influence of the electric field. When a pair of oppositely charged carriers meet, they recombine and emit light. The amount of light generated in the electroluminescent material is approximately proportional to the electric current flowing through the device, which can be increased by applying a larger electric field.
The voltage at which organic electroluminescent diodes turn on and begin to emit light is often determined by the electric field required to inject an appreciable number of charge carriers. Since most organic materials considered for electroluminescent diodes have very small intrinsic carrier densities, carrier injection from external contacts is essential, but also is often problematic due to the large electrical resistivity of the organic materials.
Similarly, the electric current flowing through the device at a particular voltage depends critically on the number of charge carriers injected from the contacts at that particular voltage. Thus, the voltage required to drive a particular electric current through the device and obtain a particular brightness can be reduced by providing improved carrier injection at the contacts. Lower operating voltages are desirable, since they allow the organic electroluminescent diode or display device to operate with lower power consumption, with a smaller power supply or longer battery lifetime, and with reduced heat dissipation.
Contacts to organic light emitting diodes are typically fabricated using inorganic materials. A low-work function metal, such as calcium, magnesium, or aluminum, is typically used for the electron-injection (cathode) contact, and a conductive transparent metal oxide, such as indium tin oxide, is often used as the hole-injecting (anode) contact. At least one of the contacts is usually transparent or semi-transparent so that the light generated in the electroluminescent material can exit the device efficiently. Indium tin oxide provides not only large optical transmittance, but also a relatively large work function which is beneficial to obtaining good hole injection from the anode contact. A low work function, such as provided by calcium, magnesium, or aluminum contacts, is beneficial to obtaining efficient electron injection from the cathode.
Many of the problems and limitations of organic light emitting devices are due to the fact that the typically used inorganic contacts usually must inject carriers into organic materials with very small intrinsic carrier densities. The problems associated with the inorganic/organic contact interfaces can be reduced by sandwiching a thin layer of a highly conductive organic contact material between the organic light emitter and the inorganic contact. This has resulted in a significant improvement in the carrier injection efficiency.
Organic contact: materials that have previously been considered include polyaniline and the phthalocyanines, such as copper phthalocyanines (CuPc) and 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA). Also, ultrathin self-assembled polymer layers have improved carrier injection properties in organic light emitters.
The improvement in carrier injection is typically credited to an improved energy band lineup at the contact interface. More specifically, the introduction of a suitable interfacial layer is believed to reduce the height of the energy barrier which the charge carriers have to surmount upon injection from the contact into the organic light emitter, resulting in lower turn-on voltage and larger current densities. That is, the organic layer is used to modify the effective work function of the inorganic contact material, either by using an organic conductor as a functional replacement for an inorganic conductor (but with a modified work function) or by using an organic interfacial layer to develop a potential drop which modifies the work function.
The present invention provides a light emitting device having an organic light emitting layer and an organic semiconductor layer that enhances carrier density or injection. These layers are interposed between first and second contact layers. A carrier transport layer can be optionally interposed between the light emitting and semiconductor layers. When used as a diode, the first and second contacts function as an anode and a cathode.
According to other embodiments of the present invention, the light emitting device is further provided with a gate contact and a gate dielectric. These embodiments function as a field effect device with the first and second contacts also functioning as a source and a drain, depending on whether the semiconductor layer is a p-type or n-type material.
The devices of the present invention have the important advantages of a much wider range of available material band gaps and work functions. The field effect device embodiments have the ability of controlling the carrier density in the organic semiconductor to control injection into the light emitter.